1. Field of the Invention
The present invention relates to an improvement in a fiber optic gyroscope (FGO) that detects a rotation to an inertial space.
2. Description of the Related Art
A typical of a substrate-type optical integrated circuit that is used in a fiber optic gyroscope (hereinafter, referred to simply as gyro) is a Y-branching or Y-propagating optical waveguide that is fabricated on an optical crystal substrate of Lithium Niobate (LiNbO3) using proton exchange method. Though two propagation modes, namely, a TE mode (Transverse Electric mode) and a TM mode (Transverse Magnetic mode) are produced in a normal optical waveguide, this optical waveguide fabricated using the proton exchange method has its inherent nature that only the TE mode is formed as a guide mode or propagation mode and the TM mode is not formed as a guide mode or propagation mode. In other words, the optical waveguide itself, fabricated on the optical crystal substrate of Lithium Niobate using the proton exchange method, is provided with a function of a polarizer having very high extinction ratio. As a result, when light from a light source is incident on the proton exchanged optical waveguide, only light wave of the TE mode is propagated, whereas light wave of the TM mode will not be propagated and will be extinguished. The above-mentioned technology is described in, for example, Japanese Patent No. 2552603 (Japanese Patent Application Public Disclosure No. 05-196471) and Japanese Patent Application Public Disclosure No. 08-029184, Japanese Patent No. 2737030 (International Publication No. WO 95/34010 corresponding to U.S. Pat. No. 5,475,772), or Proceedings of SPIE, Vol. 2292, pp. 166-176.
In the substrate-type optical integrated circuit having the function of a polarizer there may happen a phenomenon that notwithstanding a component of polarization to be extinguished, namely, light wave of the TM mode has been really leaked without coupling to light wave of the TE mode, the TM mode light wave is reflected, for example, from a bottom of the optical integrated circuit substrate so that conversion of the polarization state (mode conversion) occurs and a portion thereof is coupled to the propagation mode, or the like. Such irregular re-coupling of the extinction mode to the propagation mode apparently results in a phenomenon that extinction ratio of the polarizer is insufficient which causes an error in a detected output (angular velocity) of a fiber optic gyro, that is, a bias (degree/hour) in a detected output of a fiber optic gyro. In other words, in spite that an optical waveguide fabricated using proton exchange method should intrinsically have very high extinction ratio, there may appear a bias due to shortage of extinction.
Since such bias periodically varies in its magnitude depending upon a difference in phase of the leaked and coupled light to the TM mode, in a temperature test or the like of a fiber optic gyro, for example, it is observed as a periodically varying bias attendant upon a gradient of temperature. The reason is that in general, a difference occurs in coefficient of temperature variation of an optical path in an optical system such as an optical integrated circuit, optical fiber, or the like between polarization modes (TE mode and TM mode).
As to occurrence of such stray light in an optical crystal substrate on which an optical waveguide was fabricated using the proton exchange method, there is described in, for example, Japanese Patent No. 2737030 mentioned above. However, this prior art describes only a phenomenon that the extinguished light wave of the TM mode is reflected from a bottom of the optical waveguide substrate of LiNbO3 or LiTaO3 and a portion of the reflected light wave propagates without mode conversion and exits from the output end of the proton exchanged optical waveguide through an optical fiber connected thereto and hence it couples to the subsequent light wave of the TE mode.
On the contrary, the present invention aims at a phenomenon that stray light of a TM mode is subjected to irregular reflections or the like in an optical waveguide substrate so that it is mode-converted to a TE mode, and that the phase-delayed stray light of the TE mode propagates and exits from the output end of the optical waveguide through an optical fiber connected thereto, that is, the phase-delayed stray light couples to the main propagation mode (TE mode) and it interferes with the subsequent light wave of the main mode, which results in an error in a detected output of a fiber optic gyro. An object of the present invention is to suppress the influence of re-coupling of the stray light, particularly a component thereof that has been mode-converted, in the proton exchanged optical waveguide.
FIG. 1 shows a construction of a prior art closed loop type fiber optic gyro. Light emitted from a light source 10 propagates and is incident on an optical integrated circuit 14 through a first optical fiber 11, a fiber optic coupler 12 and a second optical fiber 13 in series. The first optical fiber 11 is used to couple between the light source 10 and the fiber optic coupler 12 and is formed by a polarization maintaining optical fiber in this example. The second optical fiber 13 is used to couple between the fiber optic coupler 12 and the optical integrated circuit 14 and is formed by a polarization maintaining optical fiber in this example. In this example, the fiber optic coupler 12 is fabricated by two polarization maintaining optical fibers, and the optical integrated circuit 14 is provided with a Y-branching optical waveguide 15 that is fabricated on an optical crystal substrate of Lithium Niobate (LiNbO3) using the proton exchange method and two optical or light modulators 16 and 17.
Light entered into the Y-branching optical waveguide 15 is branched into two light waves, namely, a first light wave and a second light wave, and the first light wave propagates and is incident on an fiber optic coil 20 through a third optical fiber 18 to propagate through the fiber optic coil 20 in the clockwise direction (hereinafter, referred to as CW direction). The second branched light wave propagetes and is incident on the fiber optic coil 20 through a fourth optical fiber 21 to propagate through the fiber optic coil 20 in the counterclockwise direction (hereinafter, referred to as CCW direction). In this example, the third and fourth optical fibers 18 and 21 are formed by polarization maintaining optical fibers, respectively. The inherent axes of these polarization maintaining optical fibers 18 and 21 are spliced to the output ends of the Y-branching optical waveguide 15 in the axis rotation method with each polarization maintaining optical fiber having its inherent axes placed at an angle of 45 degrees. As a result, both of the polarization maintaining optical fibers 18 and 21 function as depolarizers, respectively. The fiber optic coil 20 is formed by a single mode optical fiber.
A phase difference is produced between the light wave propagating through the fiber optic coil 20 in the CW direction and the light wave propagating through the fiber optic coil 20 in the CCW direction as the fiber optic coil 20 rotates. These light waves are entered into the Y-branching optical waveguide 15 and are coupled to each other so that an interference light is produced. The interference light is entered into the fiber optic coupler 12 and sent to a photodetector 25 which, in turn, outputs an electric signal corresponding to an intensity of the interference light. The electric signal outputted from the photodetector 25 is supplied to a detection circuit 26.
The optical modulators 16 and 17 are used to make the detection sensitivity of the gyro high. The first optical modulator 16 is located on one of the branched optical waveguides of the Y-branching optical waveguide 15 and the second optical modulator 17 is located on the other branched optical waveguide. To the second optical modulator 17 is supplied a phase modulation signal (for example, a sinusoidal wave) from a phase modulation circuit 27, thereby to phase-modulate the light wave propagating through the other branched optical waveguide. At the same time, the phase modulation circuit 27 supplies a synchronizing signal to the detection circuit 26 which, in turn, synchronously detects an electric signal outputted from the photodetector 25.
A detection output that is outputted from the detection circuit 26 and corresponds to an inputted angular velocity is supplied to a feedback signal generator circuit 28. The feedback signal generator circuit 28 generates a feedback signal corresponding to the magnitude of the inputted detection output and supplies it to the first optical modulator 16 to control such that the detection output from the detection circuit 26 comes to zero. An output signal of the fiber optic gyro (FOG) is obtained from the feedback signal generated from the feedback signal generator circuit 28. The construction and operation of such closed loop type fiber optic gyro are already known, and are described in, for example, the aforementioned Proceedings of SPIE, Vol. 2292, pp. 166-176. In addition, the construction and operation of an open loop type fiber optic gyro are already known, and are described in, for example, the afore-mentioned Japanese Patent Application Public Disclosure No. 08-029184. Therefore, detailed explanation thereof will be omitted here.
The reason that the optical path from the light source 10 via the fiber optic coupler 12 to the optical integrated circuit 14 is formed by a polarization maintaining optical fiber is such that the output light from the light source 10 is normally in the state of partially polarized light and if there is any fluctuation of polarization in the optical path from the light source 10 via the fiber optic coupler 12 to the Y-branching optical waveguide 15 of the optical integrated circuit 14, light wave having, correlativity between TE mode and TM mode in the Y-branching optical waveguide 15 is excited so that the TM mode becomes coherent state to the TE mode. In case of the optical waveguide of Lithium Niobate fabricated by using the proton exchange method, the TM mode comes to the leaky mode toward the outside of the optical waveguide and is extinguished. However, in case the leaky mode again couples to the optical waveguide in the optical integrated circuit substrate, if the TE mode and the TM mode are in the coherent state as mentioned above, they interfere with each other to bring about an error (drift) in the output of the fiber optic gyro. Because of such problem, it is a general procedure that the optical path from the light source 10 via the fiber optic coupler 12 to the optical integrated circuit 14 is not formed by a single mode optical fiber but is entirely formed by a polarization maintaining optical fiber.
In case the optical path from the light source 10 via the fiber optic coupler 12 to the optical integrated circuit 14 is formed by a single mode optical fiber (the fiber optic coupler 12 is also formed by single mode optical fibers), an example of the output of the fiber optic gyro is shown in FIG. 2, and in case the optical path from the light source 10 via the fiber optic coupler 12 to the optical integrated circuit 14 is entirely formed by a polarization maintaining optical fiber like the prior art shown in FIG. 1, an example of the output of the fiber optic gyro is shown in FIG. 3. Further, in both examples, a polarization maintaining optical fiber of its length 2L (length L will be described later) and a polarization maintaining optical fiber of its length 4L are inserted in the optical paths between the respective input/output ends of the Y-branching optical waveguide 15 of the optical integrated circuit 14 nearer the fiber optic coil 20 and the corresponding input/output ends of the fiber optic coil 20 formed by a single mode optical fiber, respectively.
As is clear by comparing FIGS. 2 and 3, in case all of the optical path from the light source 10 via the fiber optic coupler 12 to the input/output end nearer the light source of the Y-branching optical waveguide 15 of Lithium Niobate is formed by a polarization maintaining optical fiber having no fluctuation of polarization, it is understood that any drift (bias) is suppressed.
Incidentally, in the construction of the prior art fiber optic gyro shown in FIG. 1, as is easily understood from FIG. 3, periodical variation of the bias attendant upon a gradient of temperature is not observed so much. The reason is that all of the optical system from the light source 10 to the optical integrated circuit 14 including the fiber optic coupler is formed by a polarization maintaining optical fiber.
The polarization maintainability of the optical system is based on double refraction and in a polarization maintaining optical fiber, due to its double refraction, there is a difference in transmission rate between two orthogonal linearly polarization modes. That is, there exist a slow axis (TM mode) and a fast axis (TE mode) as a polarization axis, and in case of a light source such as a super luminescence diode (SLD) that is used in a general fiber optic gyro, as to a coherence of emitted light therefrom, if the emitted light propagates through a polarization maintaining optical fiber by a distance of several ten cm or so, a group delay between both polarization modes fully exceeds the coherence of light, and hence any coherence is already disappeared between lights of both polarization modes. Accordingly, as described above, even if a stray light to be extinguished is irregularly re-coupled in the optical integrated circuit, no bias occurs if there is no coherence between the stray light and light of the propagation mode, and also there is no periodicity depending upon temperature based on the phase relationship.
As is well known, the polarization maintaining optical fiber is expensive and especially, the fiber optic coupler fabricated by polarization maintaining optical fibers is very costly because a manufacturing process in which it is fabricated by matching the polarization axes of two optical fibers with each other is very complicated. For this reason, it comes to an obstacle to reduce the cost of production.
Recently, in a fiber optic gyro, in order to reduce the cost thereof, it has been carried out to fabricate a fiber optic coil part by use of a single mode optical fiber. This can be achieved by a technique that gives a function of depolarizer to a fiber optic coil consisting of a single mode optical fiber to maintain the polarization state of light propagating through the coil in constant and that suppress a variation of an output bias based on a variation of the polarization state. In one implementation example thereof, it is configured that to both ends of a fiber optic coil consisting of a single mode optical fiber are connected polarization maintaining optical fibers each of which has its length that results in a sufficient group delay to light propagating therethrough, respectively, and each polarization maintaining optical fiber is connected to the corresponding optical waveguide of the optical integrated circuit with their polarization axes spliced in the axis rotation method with an angle of 45 degrees between them (that is, a function of depolarizer is given to each of the polarization maintaining optical fibers), and that while light wave entering into the fiber optic coil from the corresponding optical waveguide of the optical integrated circuit propagates through the associated polarization maintaining optical fiber, polarization is depolarized. Moreover, in this implementation example, in order to prevent effects of the group delays in the polarization maintaining optical fibers connected respectively to both ends of the fiber optic coil consisting of a single mode optical fiber from being cancelled each other while light to be transmitted in the CW direction or in the CCW direction propagates through both of the polarization maintaining optical fibers, a difference is provided in their lengths of the two polarization maintaining optical fibers, this difference in length being greater than unit length of the polarization maintaining optical fiber that is required to depolarize polarization of light propagating through the polarization maintaining optical fiber of unit length. Further, in a fiber optic gyro, as to fabrication of the fiber optic coil by use of a single mode optical fiber in order to reduce the cost of the gyro, there are described in, for example, Japanese Patent Application Public Disclosure No. 06-066572, U.S. Pat. No. 5,187,757, and Japanese Paten No. 2514530 (Japanese Patent Application Public Disclosure No. 05-322590).
In order to fabricate the optical system of a fiber optic gyro inexpensively, the inventors have performed various experiments in which the optical system from the light source to the optical integrated circuit has been replaced by an inexpensive single mode optical system instead of the expensive prior art polarization maintaining optical system. In such experiments, they have confirmed that by merely replacing the polarization maintaining optical system by a single mode optical system, an output bias occurs due to re-coupling of a stray light described above, because coherence between two polarization modes is not disappeared.